This is a proposal to isolate and characterize the gene for OKM5 and to study the function of the encoded protein. This gene encodes a human macrophage/endothelial cell surface molecule which serves as a differentiation marker for these cells and may be involved in the cytoadherence of lymphocytes to endothelial cells. This involvement may help in the understanding of how malignant cells metastasize. A cDNA clone for the antigen using the mouse transformant which I have already selected will be isolated. The sequence of this cDNA will be determined to deduce the protein structure. The function of the OKM5 molecule will be studied using immunologic blocking methods and somatic cell genetic techniques. Expression of the antigen on the surface of a variety of human promonocytic tumor cell lines is induced by a variety of factors. These inducible systems provide a model for the study of the regulation of the gene.